1. Field of the Invention
This invention relates generally to chairs for use in offices for example. More particularly, the invention relates to a chair of the type wherein at least the seat back are designed to be tiltable rearwardly against a spring or springs.
2. Description of the Prior Art
There have been proposed various types of tiltable chairs wherein at least one of the seat and the seat back is tiltable against a tilting control spring or springs. The most typical is a rocking chair wherein the seat is rearwardly tiltable together with the chair back. Such a chair enables the user to assume a relaxing posture occasionally during desk work for example.
U.S. Pat. No. 5,080,318 discloses a chair tilting control assembly wherein the supportive force of a tilting control spring device is automatically adjusted to suit the weight of any user, so that the spring device provides a stronger support for a heavier user than for a lighter user. Another chair tilting control assembly having similar automatic adjustment (though slightly different in the operating principle) is also disclosed in European Patent Application Laid-open No. 0435297.
According to either one of these patent documents, when the user sits on the chair, the seat moves downward against a weight responsive spring device which deforms proportionally to the user's weight. The downward movement of the seat results in a corresponding movement of a displacing mechanism for adjusting the supportive force of the tilting control spring device to suit the user's weight. Thus, when the user subsequently assumes a reclining posture, the seat back tilts rearwardly against the thus adjusted tilting control spring device, so that the user can enjoy comfortable reclining.
In reality, however, if the user leans on the seat back, a part of the user's weight is taken by the seat back. As a result, the seat is moved up slightly by the weight responsive spring device, thereby causing a slight returning movement of the displacing mechanism. Thus, the supportive force or ability of the tilting control spring device deviates slightly from the previously adjusted value, consequently failing to provide comfortable tilting control. Obviously, the degree of such deviation increases as the user reclines more.